1. Field of the Invention
This invention relates to a method and apparatus for processing natural gas to separate the heavier hydrocarbon components from the lighter hydrocarbon components typically found in produced natural gas. More particularly, this invention relates to the use of vortex tubes for processing natural gas streams.
2. Description of Related Art
Natural gas, as produced, typically comprises methane, ethane, propane, butane and natural gasoline. Excessive amounts of the heavier hydrocarbon components, e.g. propane, butane and natural gasoline, make the gas unsuitable for use as a gaseous fuel and create problems for gas transportation systems. In addition, the heavier components are usually more valuable when separated from the lighter components. Ethane value is sometimes higher in the gas stream and sometimes higher in the liquid stream.
It is, thus, frequently necessary to process the produced natural gas to separate the heavier hydrocarbon components from the lighter hydrocarbon components. Depending upon the component concentrations and current component values, processing natural gas can be very lucrative. Typically, component separation is carried out by the combination of refrigeration to condense the heavier components and distillation to remove the lighter components from the liquid stream. Refrigeration is often achieved by pressure reduction through a turbo-expander or a Joule-Thompson (J-T) effect control valve. The J-T effect control valve provides cooling of the gaseous stream by adiabatic expansion across a restriction.
A turbo-expander system is the most efficient and effective process for utilizing pressure drop to process produced natural gas. However, it is also expensive to construct, expensive to operate and somewhat inflexible with respect to product separation. The simpler J-T control valve system costs less to construct, costs less to operate and is more flexible with respect to product separation.
A vortex tube, also sometimes referred to as the Ranque Vortex Tube, the Hilsch Tube, the Ranque-Hilsch Tube and “Maxwell's Demon”, is a static mechanical device that takes pressurized compressible fluid and derives a hot fluid and a cold fluid at a lower pressure. First discovered by George Ranque in 1928 and later developed by Rudolf Hilsch in 1945, the mechanics of why the Ranque-Hilsch effect separates a fluid into hot and cold parts through depressurizing are largely unknown, but empirical data validate that it is a measurable, repeatable and sustainable event. In operation, the pressurized compressible fluid is injected through tangential nozzles into a vortex chamber in which the compressible fluid is simultaneously separated into a fluid stream higher in temperature than the inlet stream and a fluid stream that is cooler than the inlet stream. One widely accepted explanation of the phenomenon is that tangential injection sets the pressurized compressible fluid stream in a vortex motion. This spinning stream of compressible fluid turns 90° and passes down the hot tube in the form of a spinning shell or vortex, similar to a tornado. A valve at one end of the tube allows some of the warmed fluid to escape. That portion of the warmed fluid that does not escape is directed back down the tube as a second vortex inside the low-pressure area of the larger vortex. This inner vortex loses heat to the larger vortex and exhausts through the other end as a cold fluid stream.
It is known to those skilled in the art that the vortex tube effect can be utilized to separate a multi-component hydrocarbon stream into hot and cold streams. It has also been shown that the hot stream exists in a somewhat richer state, that is, more heavy components than the cold stream, which results in a more efficient component separation than the conventional gas/liquid separator. U.S. Pat. No. 5,976,227 teaches a device for separation of liquid from a gas-liquid mixture comprising a vortex tube through which the gas-liquid mixture flows at speeds generating centrifugal forces with acceleration greater than 50 g, which cause liquid droplets to precipitate on the interior walls of the vortex tube. Concentric channels disposed in the tube wall of the warm end of the tube provide the means for the liquid removal. An outer casing encloses the warm end tube and serves to collect the liquids, which, in turn, are directed to a standalone separator. After removal of the liquids in the warm end tube, the warm and cold gas fractions of the vortex tube are recombined into the “conditioned” gas stream. Use of the vortex in a variety of applications are exemplified by U.S. Pat. No. 5,937,654, which teaches the use of a vortex tube for mixing water with chilled air to produce snow; U.S. Pat. No. 6,082,116, which teaches a vortex heater for transferring a vortex flow's heat flux to a separate gas flow in a system including a vortex tube for the purpose of preventing pilot gas freeze up at gas pressure regulation stations; and U.S. Pat. No. 5,483,801, which teaches a process for extracting vapor from a gas stream using a vortex tube expansion. See also U.S. Pat. No. 5,950,436 and U.S. Pat. No. 5,819,541 (method of beverage cooling/heating on vehicles), U.S. Pat. No. 5,911,740 (method of heat transfer enhancement in vortex tubes), U.S. Pat. No. 5,582,012 (method of natural gas pressure reduction on city gate stations), U.S. Pat. No. 5,561,982 (method for energy separation and utilization in a vortex tube operating at pressures not exceeding atmospheric pressure), and U.S. Pat. No. 5,327,728 (method of designing a vortex tube for energy separation).
Given the state of the art with respect to natural gas processing, it is desirable to improve the efficiency of natural gas processing, reduce the costs associated with natural gas processing, and provide more flexibility in product separation than conventional natural gas processing systems.